Gas spring assembly for a vehicle suspension system

ABSTRACT

A gas spring for a vehicle suspension system includes a cylinder, a rod, and an accumulator. The rod is disposed within the cylinder, and relative movement between the rod and the cylinder changes the volume of a chamber formed between the rod and the cylinder. The accumulator is in gaseous communication with the chamber such that a continuous body of gas extends between the chamber and the accumulator.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of application Ser. No. 12/872,782, filed Aug.31, 2010, which is incorporated herein by reference in its entirety.

BACKGROUND

The present application relates to suspension systems for vehicles. Morespecifically, the present application relates to a gas spring for asuspension system.

SUMMARY

One embodiment of the invention relates to a gas spring for a vehiclesuspension system. The gas spring includes a cylinder, a rod, and anaccumulator. The rod is disposed within the cylinder, and relativemovement between the rod and the cylinder changes the volume of achamber formed between the rod and the cylinder. The accumulator is ingaseous communication with the chamber such that a continuous body ofgas extends between the chamber and the accumulator.

Another embodiment of the invention relates to a gas spring for avehicle suspension system. The gas spring includes a cylinder, a rod, anaccumulator, and a sensor. The rod is disposed within the cylinder, andrelative movement between the rod and the cylinder changes the volume ofa chamber formed between the rod and the cylinder. The accumulator is ingaseous communication with the chamber such that a continuous body ofgas extends between the chamber and the accumulator. The sensor iscoupled to at least one of the cylinder and the rod, and the sensor isconfigured to provide a signal indicative of a ride height of thevehicle suspension system based upon the relative position of the rodand the cylinder.

Yet another embodiment of the invention relates to a vehicle suspensionsystem. The vehicle suspension system includes a pump and a gas spring.The gas spring includes a cylinder, a rod, and an accumulator. The rodis disposed within the cylinder, and relative movement between the rodand the cylinder changes the volume of a chamber formed between the rodand the cylinder. The accumulator is in gaseous communication with thechamber such that a continuous body of gas extends between the chamberand the accumulator.

Alternative exemplary embodiments relate to other features andcombinations of features as may be generally recited in the claims.

BRIEF DESCRIPTION OF THE FIGURES

The disclosure will become more fully understood from the followingdetailed description, taken in conjunction with the accompanyingfigures, in which:

FIG. 1 is a perspective view of an axle assembly, according to anexemplary embodiment of the invention.

FIG. 2 is a perspective view of a suspension system, according to anexemplary embodiment of the invention.

FIG. 3 is a perspective view of a gas spring in a first configuration,according to an exemplary embodiment.

FIG. 4 is a side view of the gas spring of FIG. 3 in a secondconfiguration.

FIG. 5 is a side view of a gas spring assembly, according to anexemplary embodiment of the invention.

FIG. 6 is a front view of the gas spring assembly of FIG. 5.

FIG. 7 is a sectional view of the gas spring assembly of FIG. 6, takenalong line 7-7 of FIG. 7.

FIG. 8 is a detail view of a portion of the gas spring assembly of FIG.7, taken along line 8-8 of FIG. 7.

FIG. 9 is a detail view of a portion of the gas spring assembly of FIG.7, taken along line 9-9 of FIG. 7.

FIG. 10 is a graphical comparison of force versus displacement for asingle-stage gas spring and a two-stage gas spring based upon simulationdata, according to an exemplary embodiment of the invention.

FIG. 11 is a schematic diagram of an accumulator, according to anexemplary embodiment of the invention.

FIG. 12 is a schematic diagram of an accumulator, according to anotherexemplary embodiment of the invention.

DETAILED DESCRIPTION

Before turning to the figures, which illustrate the exemplaryembodiments in detail, it should be understood that the presentapplication is not limited to the details or methodology set forth inthe description or illustrated in the figures. It should also beunderstood that the terminology is for the purpose of description onlyand should not be regarded as limiting.

According to an embodiment, a vehicle may include a body supported by asuspension system (see, e.g., suspension system 218 as shown in FIG. 1).In some embodiments, the vehicle may be a military vehicle. In otherembodiments, the vehicle may be a utility vehicle, such as a fire truck,a tractor, construction equipment, or a sport utility vehicle. Thevehicle may be configured for operation on both paved and rough,off-road terrain. As such, the suspension system may be correspondinglyconfigured to support the weight of the vehicle while providingcomfortable ride quality on both paved and rough, off-road terrain. Insome embodiments, the suspension system is configured to change the rideheight of the vehicle by lifting or lowering the body of the vehiclewith respect to the ground.

Referring to FIG. 1, an axle assembly 210 is configured for use with thevehicle. According to an exemplary embodiment, the axle assembly 210includes a differential 212 connected to half shafts 214, which are eachconnected to a wheel end assembly 216. The wheel end assembly 216 is atleast partially controlled (e.g., supported) by a suspension system 218,which includes a spring 220, a damper 222, an upper support arm 224, anda lower support arm 226 coupling the wheel end assembly 216 to thevehicle body or part thereof (e.g., chassis, side plate, hull).

According to an exemplary embodiment, the differential 212 is configuredto be connected with a drive shaft of the vehicle, receiving rotationalenergy from a prime mover of the vehicle, such as a diesel engine. Thedifferential 212 allocates torque provided by the prime mover betweenhalf shafts 214 of the axle assembly 210. The half shafts 214 deliverthe rotational energy to the wheel-end assemblies 216 of the axleassembly 210. The wheel end assemblies 216 may include brakes, gearreductions, steering components, wheel hubs, wheels, and other features.As the vehicle travels over uneven terrain, the upper and lower supportarms 224, 226 at least partially guide the movement of each wheel endassembly 216, and a stopper 228 provides an upper bound to movement ofthe wheel end assembly 216.

Referring to FIG. 2, according to an exemplary embodiment the suspensionsystem 218 includes one or more high-pressure gas components, where thespring 220 is a high-pressure gas spring 220. In some embodiments, thesuspension system further includes at least one high-pressure gas pump230. In some such embodiments, the suspension system 218 includesseparate high-pressure gas pumps 230 associated with each spring 220 anddamper 222 set. In preferred embodiments, the gas of the pump 230,spring 220, and damper 222 includes (e.g., is at least 90%, at least95%) an inert gas such as nitrogen, argon, helium, etc., which may bestored, provided, or received in one or more reservoirs (e.g., centralreservoir, tank) (not shown).

During operation, the pump 230 selectively provides gas, under pressure,to the high-pressure gas spring 220 and/or to reservoirs, tanks,accumulators, or other devices. In some contemplated embodiments, two ormore high-pressure gas dampers 222 of the vehicle are cross-plumbed vialines 232 (e.g., hydraulic lines) connecting dampers 222 on oppositesides of the axle assembly 210, between dampers 222 in a “walking beam”configuration for a tandem axle, or between dampers 222 on separate axleassemblies of the vehicle (e.g., between dampers located front-to-back,or diagonally located with respect to each other).

Referring to FIGS. 3-4, a gas spring 310 includes a cylinder 312 coupledto a rod 314 (FIG. 4). The cylinder 312 has a cap end 316, a rod end318, and a side wall 320 (e.g., cylindrical side wall) extending betweenthe cap and rod ends 316, 318. A chamber (see, e.g., chamber 418 asshown in FIG. 7) is formed between the cylinder 312 and the rod 314—suchas interior to the cylinder 312, between the cap end 316, the side wall320, and the rod 314, which extends through the rod end 318 of thecylinder 312. Nitrogen or another gas held in the chamber compresses orexpands in response to relative movement between the rod 314 and thecylinder 312 to provide the receipt, storage, or release of potentialenergy by the gas spring 310.

The rod 314 is configured to translate with respect to the cylinder 312.According to an exemplary embodiment, the rod 314 is coupled to orcomprises a piston (see, e.g., rod 414 as shown in FIG. 7; e.g., rodend, plunger) that forms a wall of the chamber. When the rod 314translates relative to the cylinder 312, the piston changes the volumeof the chamber, compressing the gas in the chamber or allowing the gasto expand. The gas in the chamber resists compression, providing a forcethat is a function of the compressibility of the gas, the area of thepiston, the volume and geometry of the chamber, and the current state(e.g., initial pressure) of the gas, among other factors. As such, thegas spring 310 receives potential energy, stored in the gas, as the gasis compressed and releases the potential energy as the gas expands.

The cylinder 312 of the gas spring 310 is preferably cylindrical due tostructural benefits associated with cylindrical pressure vessels.However, in other contemplated embodiments, the cylinder 312 may besubstituted for a body having another geometry. In some contemplatedembodiments, the chamber may be formed in, or at least partially formedin the rod 314. In one such embodiment, the chamber spans both thecylinder 312 and at least a portion of the interior of the rod 314.

In some embodiments, the gas spring 310 includes at least one port 322(e.g., aperture, inlet) that may be opened to allow gas (e.g., inertgas) to be provided to or from the chamber. The chamber of the gasspring is substantially sealed when the port 322 is not open. In someembodiments, the port 322 may be coupled to an accumulator (see, e.g.,accumulator 416 as shown in FIG. 5), to a pump (see, e.g., pump 230 asshown in FIG. 2), or to one or more reservoirs (not shown). In someembodiments, the spring 310 includes separate ports associated with theaccumulator and the pump.

In some embodiments, the gas spring 310 further includes at least oneport 324 that may be opened to allow a pressurized reservoir of a higheror a lower pressure (see generally accumulator 416 as shown in FIG. 5)to be coupled to the gas spring 310. Coupling the higher pressurereservoir to the gas spring 310 increases the pressure in the gas spring310, causing the gas spring 310 to expand and increasing the ride heightof the axle assembly. Conversely, coupling the lower pressure reservoirto the gas spring 310 decreases the pressure in the gas spring 310,causing the gas spring 310 to contract and decreasing the ride height ofthe axle assembly. In some embodiments, the spring 310 includes separateports 324 for providing hydraulic fluid to the internal volume and forreceiving hydraulic fluid from the internal volume.

In other contemplated embodiments, the gas spring 310 is coupleddirectly to a pump (see, e.g., pump 230 as shown in FIG. 2), to increaseor decrease pressure in the gas spring 310 corresponding to a desiredride height. In still another contemplated embodiment, a gas springfurther includes at least one port that may be opened to allow hydraulicfluid (e.g., oil) to be provided to or from an internal volume (see,e.g., internal volume 432 as shown in FIG. 8) of the gas spring. Theinternal volume for hydraulic fluid is separated from the chamber forgas. In such contemplated embodiments, adding or removing of hydraulicfluid from the internal volume changes the overall length of the gasspring for different ride heights of the suspension system. Howeverusing pressurized gas to change the length of the gas spring 310 may bepreferable in some embodiments because of reduced losses (e.g.,friction, drag) associated with a flow of gas (e.g., nitrogen) comparedto hydraulic fluid (e.g., oil).

Referring now to FIGS. 5-9, a gas spring assembly 410 includes acylinder 412 coupled to a rod 414, and an accumulator 416. A firstchamber 418 (FIG. 7) is formed between the cylinder 412 and the rod 414and a second chamber 420 is formed in the accumulator 416. According toan exemplary embodiment, the accumulator 416 includes a rigid exterior424 (e.g., shell, housing) and a flexible, inflatable bladder 426 withinthe rigid exterior 424. The second chamber 420 is located between therigid exterior 424 and the bladder 426. According to an exemplaryembodiment, the accumulator 416 is positioned proximate to the cylinder412 and rod 414, and the second chamber 420 of the accumulator 416 isconnected to the first chamber 418, formed between the cylinder 412 androd 414, by way of a gas transfer conduit 422. The gas transfer conduit422 may include a valve 428 (e.g., check valve, poppet) for controllingaccess between the first and second chambers 418, 420. The valve 428 mayserve to optionally disconnect the accumulator 416 from the firstchamber 418, or to optionally contain gas in the second chamber 420having a pressure exceeding or lower than gas in the first chamber 418.

In some embodiments, when the valve 428 is open, the first chamber 418is in gaseous communication with the second chamber 420 such that acontinuous body of gas extends between the two chambers 418, 420. Nointermediate hydraulic fluid or mechanical element is included totransfer energy from the first chamber 418 to the second chamber 420 orvice versa. In some such embodiments, the only hydraulic fluidassociated with the gas spring assembly 410 is a thin film between therod and cylinder that moves during compression or extension of the rod414. Use of the continuous body of gas for gaseous communication betweenthe first and second chambers 418, 420 is intended to reduce frictionallosses associated with energy transfer between the first and secondchambers 418, 420, as may otherwise occur with hydraulic or mechanicalintermediate elements. However, in other contemplated embodiments,hydraulic or mechanical intermediate elements may be used.

During use of the gas spring assembly 410, in some embodiments, thebladder 426 is inflated to an initial pressure. As the rod 414 andcylinder 412 are moved together, such as when the associated vehicledrives over a bump, gas in the chamber 418 compresses, providing a firstspring rate for the gas spring assembly 410. In such embodiments, thepressure of the gas in the first chamber 418 is communicated to theaccumulator 416 via the transfer conduit 422. If the pressure of the gascommunicated from the first chamber 418 is below the initial pressure ofthe bladder 426, the gas spring assembly 410 will respond to the bumpwith the first spring rate. However, if the pressure of the gascommunicated from the first chamber 418 exceeds the initial pressure inthe bladder 426, then the bladder 426 will compress, increasing theeffective volume of the second chamber 418, which provides a secondspring rate to the gas spring assembly 410.

In some such embodiments, a pump (see, e.g., pump 230 as shown in FIG.2) may be coupled to the bladder 426 to increase the initial pressure ofthe bladder 426 and thereby increase the threshold amount of loadingrequired to achieve compression of the bladder 426, which would increasethe loading required to initiate the second spring rate. Or gas may bereleased from the bladder 426 to decrease the threshold. As such, thevalue of the initial pressure of the bladder 426 may be set to achieve adesired responsiveness of the gas spring assembly 410. Use of the firstand second spring rates is intended to reduce peak forces on thevehicle, improving the ride quality and durability of the vehicle.Tuning of the threshold allows for adjustment of the response of the gasspring assembly 410 depending upon a particular vehicle application.

FIG. 10 includes a graphical representation 510 of spring force 512 as afunction of spring deflection 514 for a single-stage spring 516 (withoutaccumulator) and two-stage spring 518 (with accumulator) based uponsimulation data (i.e., prophetic representation). As deflection 514increases, the force 512 of the spring correspondingly increases. Forlesser loads, the relationship between deflection 514 and force 512 issubstantially direct (e.g., quadratic, but having a substantiallystraight slope). However, when loading of the spring reaches a threshold520, the spring rate (i.e., slope of the curve) of the two-stage spring518 decreases, while the spring rate of the single-stage spring 516continues along the same trajectory (e.g., quadratic curve). The pointof inflection 522 along the two-stage spring 518 curve is adjustable byincreasing or decreasing the initial pressure in the bladder.

Referring again to FIGS. 5-9, according to an exemplary embodiment, thegas spring assembly 410 includes at least one port 430 (FIG. 8) to allowhydraulic fluid to be provided to an internal volume 432 within the gasspring assembly 410. Hydraulic fluid passes through the port 430 andalong a conduit 434, which distributes the hydraulic fluid into theinternal volume 432 by way of a distribution element 436 (e.g.,perforated plate).

In some embodiments, a floating, annular piston 438 is used to separatethe hydraulic fluid in the internal volume 432 from the gas of thechamber 418. Standard or conventional hydraulic seals 440 may be usedwith respect to the annular piston 438 and port 430 of the internalvolume 432 to prevent leakage of the hydraulic fluid. In someembodiments, standard accumulator seals are used to seal the annularpiston 438. According to an exemplary embodiment, the internal volume432 surrounds at least a portion of the first chamber 418 (for gas)within the gas spring assembly 410. As such, the hydraulic seals 440serve to seal the gas within the gas spring 410.

According to an exemplary embodiment, the gas spring assembly furtherincludes a sensor 442 integrated with the gas spring assembly 410 andconfigured to sense the relative configuration of the rod 414 andcylinder 412. In some embodiments, the sensor 442 provides a signal(e.g., digital output) that is indicative of the ride height of theassociated suspension system (see, e.g., suspension system 218 as shownin FIG. 1) based upon the relative configuration of the rod 414 andcylinder 412. In contemplated embodiments, the sensor 442 includes alinear variable differential transformer (LVDT), where a shaft of theLVDT extends through the cylinder 412 to the rod 414. As the rod 414 andcylinder 412 move relative to one another, the shaft of the LVDTprovides a signal (e.g., inductive current) that is a function of themovement of the shaft.

Referring now to FIG. 11, an accumulator 610 includes a cylinder 612having a first section 614 and a second section 616. In someembodiments, the first section 614 has a narrower cross section than thesecond section 616. The accumulator 610 further includes a pistonassembly 618 having a first face 620 corresponding to the first section614 and a second face 622 corresponding to the second section 616. Aninlet 624 is coupled to the first section 614 and is configured to be ingaseous communication with gas from a chamber of a gas spring (see,e.g., chamber 418 as shown in FIG. 7). As gas is provided to the firstsection 614, the piston assembly 618 is moved, compressing a separatebody of gas 626 in the second section 616. Compression of the secondbody of gas 626 receives potential energy, stored in the compressed gas.

In some embodiments, the accumulator 610 additionally includes atransfer tube 628 extending between the first and second sections 614,616. The transfer tube 628 allows for controlled transfer of gas fromthe second section 616 to the first section 614, or vice versa. Arestrictor 630 or valve may be positioned along the transfer tube 628 tocontrol the flow of gas through the transfer tube 628. FIG. 12 shows analternate embodiment of an accumulator 710 where a transfer tube 712 andrestrictor 714 or valve is integrated with a piston assembly 716.

In some embodiments that include the transfer tube 628, 712, the twosections 614, 616 of the accumulator 610 are in gaseous communication atequilibrium (e.g., steady state). Equal pressure acts on both sides ofthe piston assembly 618, 716. But, due to the unequal cross-sections, anet force biases the piston assembly 618, 716 toward the first section614. At standard operating pressures of the gas spring, the equilibriumpressure supplies a net force sufficient to overcome forces of gravityand friction acting on the piston assembly 618, 716.

During an impulse loading event, the spring compresses and rapidlycommunicates increased gas pressure to the first section 614 of theaccumulator 610. However, due in part to the setting of the restrictor630 and drag in the transfer tube 628, 712, the pressure in the secondsection 616 of the accumulator 610 does not increase as rapidly. Assuch, with a sufficient pressure differential between the first andsecond sections 614, 616, the piston assembly 618, 716 moves from theinitial position. The volume of the first section 614 increases and thevolume of the second section 616 decreases, compressing the gas in thesecond section 616, which results in a different spring rate (see, e.g.,point of inflection 522 as shown in FIG. 10) for the overall gas springassembly.

According to an exemplary embodiment, the second spring rate andthreshold at which the bias of the piston assembly 618, 716 is overcomeis tunable by changing the area ratio of the piston assembly 618, 716(i.e. chamber cross-sections). In some contemplated embodiments, thesetting of the restrictor 630 controls damping to the accumulator 610and overall gas spring assembly, which may be used with or without aseparate damper (see, e.g., damper 222 as shown in FIG. 1).

In other contemplated embodiments, the separate body of gas 626 in thesecond section 616 may be set to an initial pressure, such as by a pump(see, e.g., pump 230 as shown in FIG. 2), to bias the piston assembly618 to an initial position. The pressure of the second body of gas 626holds the piston assembly 618 in the initial position until the force ofgas supplied to the first section 614 via the inlet 624 exceeds theforce provided by the initial pressure of the separate body of gas 626in the second section 616.

The construction and arrangements of the gas spring assembly, as shownin the various exemplary embodiments, are illustrative only. Althoughonly a few embodiments have been described in detail in this disclosure,many modifications are possible (e.g., variations in sizes, dimensions,structures, shapes and proportions of the various elements, values ofparameters, mounting arrangements, use of materials, colors,orientations, etc.) without materially departing from the novelteachings and advantages of the subject matter described herein. Someelements shown as integrally formed may be constructed of multiple partsor elements, the position of elements may be reversed or otherwisevaried, and the nature or number of discrete elements or positions maybe altered or varied. The order or sequence of any process, logicalalgorithm, or method steps may be varied or re-sequenced according toalternative embodiments. Other substitutions, modifications, changes andomissions may also be made in the design, operating conditions andarrangement of the various exemplary embodiments without departing fromthe scope of the present invention.

What is claimed is:
 1. A gas spring for a vehicle suspension system,comprising: a cylinder; a rod disposed within the cylinder, whereinrelative movement between the rod and the cylinder changes the volume ofa chamber formed between the rod and the cylinder; and an accumulator ingaseous communication with the chamber such that a continuous body ofgas extends between the chamber and the accumulator.
 2. The gas springof claim 1, wherein compression of gas in the chamber at least partiallyprovides a first spring rate for deflections of the gas spring below athreshold, and wherein compression of gas in the chamber at leastpartially provides a second spring rate for deflections of the gasspring above the threshold.
 3. The gas spring of claim 2, wherein thefirst spring rate is greater than the second spring rate.
 4. The gasspring of claim 3, wherein the accumulator comprises a rigid exteriorthat defines an accumulator volume and a movable component internal tothe rigid exterior.
 5. The gas spring of claim 4, wherein the movablecomponent separates the accumulator volume into a first portion and asecond portion, wherein the first portion is in gaseous communicationwith the chamber and the threshold is a function of pressure within thesecond portion.
 6. The gas spring of claim 5, wherein the threshold istunable by controlling the accumulator.
 7. The gas spring of claim 6,the accumulator further comprising a port in the rigid exteriorconfigured to facilitate changing the pressure within the secondportion.
 8. The gas spring of claim 5, wherein the movable component isa diaphragm.
 9. The gas spring of claim 2, wherein the accumulatorcomprises: an auxiliary cylinder having an auxiliary chamber formedtherein; and a piston assembly movable within the auxiliary cylinder tochange the volume of the auxiliary chamber, wherein the piston assemblyis biased to an initial position within the auxiliary cylinder thatcorresponds to an initial volume of the auxiliary chamber, wherein whendeflection of the gas spring exceeds the threshold, gas moves the pistonassembly to increase the volume of the auxiliary chamber.
 10. The gasspring of claim 9, wherein the auxiliary cylinder includes a firstsection and a second section having different diameters, wherein theauxiliary chamber is formed in the first section, and wherein the pistonassembly includes a first face corresponding to the first section and asecond face corresponding to the second section.
 11. The gas spring ofclaim 10, wherein the second section is configured to support gas, andwherein the second face of the piston assembly compresses the gas of thesecond section when the piston assembly is moved from the initialposition.
 12. The gas spring of claim 11, wherein the diameter of thefirst section of the auxiliary cylinder is smaller than the diameter ofthe second section.
 13. The gas spring of claim 12, wherein theaccumulator further comprises a gas transfer tube coupling the firstsection and the second section.
 14. The gas spring of claim 13, whereinthe gas transfer tube includes at least one of a restrictor and a valvefor controlling flow of gas between the first section and the secondsection.
 15. A gas spring for a vehicle suspension system, comprising: acylinder; a rod disposed within the cylinder, wherein relative movementbetween the rod and the cylinder changes the volume of a chamber formedbetween the rod and the cylinder; an accumulator in gaseouscommunication with the chamber such that a continuous body of gasextends between the chamber and the accumulator; and a sensor coupled toat least one of the cylinder an the rod, wherein the sensor isconfigured to provide a signal indicative of a ride height of thevehicle suspension system based upon the relative position of the rodand the cylinder.
 16. The gas spring of claim 15, wherein the sensorcomprises a linear variable differential transformer that includes ashaft extending through the cylinder to the rod.
 17. A vehiclesuspension system, comprising: a pump; and a gas spring, comprising: acylinder; a rod disposed within the cylinder, wherein relative movementbetween the rod and the cylinder changes the volume of a chamber formedbetween the rod and the cylinder; and an accumulator in gaseouscommunication with the chamber such that a continuous body of gasextends between the chamber and the accumulator.
 18. The vehiclesuspension system of claim 17, wherein the pump provides pressurized gasto the cylinder to change the length of the gas spring for differentride heights of the vehicle suspension system.
 19. The vehiclesuspension system of claim 18, further comprising: a higher-pressurereservoir of gas; and a lower-pressure reservoir of gas, wherein thepump provides the pressurized gas to the gas spring by way of at leastone of the higher-pressure reservoir of gas and the lower-pressurereservoir of gas, wherein coupling of the higher-pressure reservoir ofgas to the gas spring increases the ride height and wherein coupling ofthe lower-pressure reservoir of gas to the gas spring decreases the rideheight.
 20. The vehicle suspension system of claim 18, furthercomprising a sensor coupled to at least one of the cylinder an the rod,wherein the sensor is configured to provide a signal indicative of aride height of the vehicle suspension system based upon the relativeposition of the rod and the cylinder.